1. Field of the Invention
The present invention relates to hydroxyalkyl methylcelluloses having excellent thermoreversible gel strength and improved solubility.
2. Description of the Related Art
In hydroxypropyl methylcellulose obtained by ether-substitution of methyl groups and hydroxypropyl groups to cellulose, methoxyl groups are localized in the molecule relative to the cellulose chain. Hydroxypropyl methylcellulose has therefore “thermoreversible gelation properties”. Described specifically, when an aqueous solution of hydroxypropyl methylcellulose is heated, hydrophobic hydration of the methoxyl groups localized in the molecule occurs and it turns into a hydrous gel. When the resulting gel is cooled, on the other hand, hydrophobic hydration decreases, whereby the gel returns to the original aqueous solution. Because of such thermoreversible gelation properties, the aqueous solution shows excellent shape retention even after heating. For example, when hydroxyalkyl methylcellulose is used as a binder for extrusion of ceramics, the hydroxyalkyl methylcellulose dissolved in water is mixed, kneaded with ceramic particles, formed into a certain shape, and dried with heating wherein the hydroxyalkyl methylcellulose turns into a gel by heating. When the gelled portion has high strength, defects such as cracks caused by shrinkage strain during drying can be prevented. Hydroxyalkyl methylcelluloses are therefore used exclusively as a binder for extrusion of ceramics.
Methylcellulose having no hydroxyalkoxyl groups has excellent thermoreversible gelation performance. The thermoreversible gel strength of the methylcellulose is determined by placing a 2.5% by weight aqueous solution of it in a constant temperature bath of 80° C. so as to cause thermoreversible gelation after 15 minutes; inserting a cylindrical rod having a diameter of 15 mm downward into the gel at a rate of 5 cm/min; and measuring a maximum load (g) applied to the cylindrical rod when it is inserted into 2 cm inside of the gel. The thermoreversible gel strength is obtained by dividing the maximum load (g) by the cross-sectional area of the rod. The thermoreversible gel strength thus determined is as high as from 500 to 700 g/cm2. When such methylcellulose is used for extrusion of ceramic particles, followed by drying with heating, cracks due to shrinkage strain during drying can be minimized. Even if the amount of methylcellulose added to the ceramic is reduced, defects caused by cracks during drying can be reduced.
When a molar substitution of hydroxyalkoxyl groups of more than 0.1 is introduced into methylcellulose, the resulting hydroxyalkyl methylcellulose has thermoreversible gel strength of 100 g/cm2 or less. For example, hydroxypropyl methylcellulose having a molar substitution of hydroxypropoxyl groups of 0.15 and a substitution degree of methoxyl groups of 1.8 renders thermoreversible gel strength of 30 g/cm2. Hydroxyethyl methylcellulose having a molar substitution of hydroxyethoxyl groups of 0.15 and a substitution degree of methoxyl groups of 1.8 renders thermoreversible gel strength of 25 g/cm2. This means that when high thermoreversible gel strength is required, methylcellulose having no hydroxyalkoxyl groups introduced is preferred. In order to dissolve methylcellulose having no hydroxyalkoxyl groups introduced in water, however, a water temperature has to be adjusted to 10° C. or less. Methylcellulose insoluble in water cannot render its original thermoreversible gel strength and therefore becomes practically useless. It is therefore difficult to use methylcellulose having no hydroxyalkoxyl groups introduced.
Japanese Patent Application Examined Publication No. 62-059074/1987 discloses that hydroxyalkyl methylcelluloses having a molar substitution of hydroxyalkoxyl groups of 0.02 to 0.13 can dissolve in water after a predetermined time even when the temperature is set at approximately 30° C., indicating that dissolution is possible without the dissolution temperature decreased. Further, Japanese Patent Application Examined Publication No. 62-059074/1987 discloses that the thermoreversible gelation temperature of methylcellulose can be raised by introduction of hydroxyalkoxyl groups therein.